Client instances can share config because `default_config` contains mutable values

[jonprindiville]

I have never (knowingly) been tripped up by it, but I noticed this while reviewing #137...

gapipy.client.default_config is basically used as a default argument to client initialization and some of its content are (mutable) dicts. Because of that it's possible to accidentally "share" data there -- you might have some surprises if you instantiate multiple clients in the same process with different configs.

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To demonstrate...

>>> from gapipy import Client

>>> # Let's make a client and enable connection pooling
>>> c1 = Client(connection_pool_options={"enable": True})
>>> c1.connection_pool_options["enable"]
True

>>> # Ok, let's make another WITHOUT connection pooling
>>> c2 = Client(connection_pool_options={"enable": False})
>>> c2.connection_pool_options["enable"]
False

>>> # Cool, now let's see what that first client is up to, it was True to begin with...
>>> c1.connection_pool_options["enable"]
False

>>> c1.connection_pool_options is c2.connection_pool_options
True

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I suspect the crux of the issue is probably twofold:

• new client instances get direct references to stuff in default_config and some of those things are mutable
• Client.__init__ explicitly mutates the default connection_pool_options dict when some extra connection pool configs have been passed

I bet you could have a similar issue with the global http headers config, except that Client.__init__ doesn't mutate that one you'd have to have a series of events like:

• instantiate a client
• instantiate another client
• mutate one of their client.global_http_headers and the other client is affected because they both shared the same default value

(Pretty sure I had a hand in both of those connection-pool-options and global-http-headers things 🤦 oops)

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I haven't taken a run at writing tests for it yet, but I wonder if the fix is simply:

• update get_config to copy.deepcopy (or similar) the value it gets from default_config before returning it, and
• use get_config (instead of direct dict access) when getting "connection_pool_options" out of default_config

We could also take other approaches like:

• Client.__init__ should deepcopy the default dict before yanking stuff out, or
• the default dict should be returned from a function instead of existing at at module level, or
• something else entirely 🤷